Method of producing strings for ball rackets, particularly for tennis rackets, and a string produced by this method

ABSTRACT

In the method for manufacturing strings, a plurality of narrow strip layers (22) of synthetic material are successively and coaxially helically wound on the cylindrical surface of a continuous core. The winding (3) thus produced is conveyed hot to a welding area (7) wherein the narrow strips (22) of synthetic material are mutually assembled by welding. The temperature of the welding area is adjusted high enough so that a welding between the narrow strips (22) takes place without substantially decreasing the breaking strength of the narrow strips (22) by such heat treatment. The appropriate materials for the narrow strips are olefines such as homopolymer polypropylene or terpolymer polypropylene-polyethylene-dienes. To reduce the tendency to flow, the material forming the narrow strips may contain a nucleation agent. The characteristics of the strings thus manufactured are similar to those of gut strings.

TECHNICAL FIELD

The invention relates to a method of producing a string for ballrackets, particularly for tennis rackets, wherein a plurality of windinglayers of helically wound plastics sheet bands are applied to acontinuously fed core, one over the other, along at least approximatelyconcentric cylindrical surfaces, and the winding layers are joined toone another and to the core. The invention further relates to a stringproduced by the method according to the invention.

PRIOR ART

A method of the kind described above is known from U.S. Pat. No.3,024,589. In this method a continuously fed nylon thread, serving ascore, is for example impregnated with a liquid binder and then woundhelically in two superimposed winding layers, wound in oppositedirections, and the excess binder is stripped off. The resulting wetstrand then passes through heated tubes, is thereby dried, and finallyis drawn off and wound up as a finished string. According tomodifications of the method additional pairs of winding layers may beapplied to the wet strand, while however before each addition of a pairof winding layers the strand must be once again impregnated with theliquid binder. Instead of nylon threads, extruded bands may also be usedas winding material.

The string known from the abovementioned U.S. Pat. No. 3,024,589 shouldhave properties approximating to those of the customary gut strings usedfor stringing ball rackets. For the production of these gut strings thestarting material is sheep or cattle gut, which is cut into narrow bandsand subjected to treatment by chemical processes. A plurality of thesenarrow bands are then twisted together to form the strings. These gutstrings now have the property that the dependence of their extension onthe tensile force applied is substantially linear, that is to say themodulus of elasticity of the string material is substantially constant,and therefore is also not dependent on the prestress with which thestring was fitted on the racket.

In FIG. 1 the curve A shows, for an ordinary commercially available gutstring, the spring constant E.A. (in kN) defined by the product of themodulus of elasticity E and the cross-sectional area A of the string,plotted against the prestress F_(v) (in N) of the string. As can beseen, the value of this spring constant E.A. varies only slightly withthe prestress of the string. This gives rise to the good playingproperties of tennis rackets strung with gut.

A disadvantage of stringing with gut consists on the one hand of thedifferences in quality which are unavoidable in manufacture and whichare caused by fluctuations of the quality of the gut material used, andon the other hand of their high capacity to absorb moisture, whichbecause of the consequent considerable variation of length, for examplewith high atmospheric humidity, impairs the playing properties ofrackets strung with gut. In addition, the production of gut strings isrelatively expensive.

For some years tennis rackets have now also been strung with strings ofplastics materials. Ordinary commercially available plastics strings,which generally consist of a plastics monofilament, now have springconstant characteristics of a kind indicated, for example, by curve B inFIG. 1. In the applicable range of string prestress from 200 to 300 N,the spring constant E.A. is higher than that of comparable gut stringsand in addition increases substantially linearly with a relatively steepgradient with increasing prestress. Consequently, the deformations ofthe strings which occur when the ball hits the racket are less thanthose occurring with comparable gut strings, and the force peaks whichare required for braking a determined kinetic energy of the ball, andwhich have to be absorbed by the racket, are correspondingly higher thanin the case of gut strings.

The player therefore feels that a racket strung with plastics strings is"hard" and that, in comparison with rackets strung with gut strings, ithas a harder action, the more forcefully the strokes have to be made.

In comparison with a string made of a polyamide monofilament, acommerically available string of polyamide (nylon) of the type describedin the previously mentioned U.S. Pat. No. 3,024,589 does not have animproved spring constant characteristic and, at least in respect of thisaspect, does not come close to the properties of gut strings.

DESCRIPTION OF THE INVENTION

The object underlying the invention is that of indicating a method ofproducing strings for ball rackets, particularly for tennis rackets, inwhich a plurality of winding layers of helically wound plastics sheetbands are applied to a continuously fed core, one over the other, alongat least approximately concentric cylindrical surfaces, and the windinglayers are joined to one another, and which is less expensive than theknown method mentioned in the prior art and leads to strings which havea similar flat spring tension characteristic to that of gut strings,while however it does not entail the disadvantages inherent in gutstrings.

The object underlying the invention is achieved by the method accordingto the invention, wherein the plastics sheet bands used are ofmonoaxially stretched plastics materials, and wherein after the windinglayers have been wound over the core the resulting wound assembly ispassed, while under tensile stress, through a welding zone in which theplastics sheet bands are welded together at a raised temperature. Thetemperature in the welding zone is advantageously set at such a heightthat, although welding union takes place between the plastics sheetbands, this temperature treatment does not substantially reduce thebreaking strength σ_(R) of the monoaxially stretched plastics sheetbands used in the winding layers, advantageously ensuring that, if thestring is at least substantially composed of plastics sheet bands of auniform material, the temperature in the welding zone is so adjustedthat the reduction of the breaking strength of the plastics sheet bandsduring the welding operation leads to a breaking strength of thefinished strings which is not more than 20%, and advantageously not morethan 15%, below the breaking strength of the plastics sheet bands used.

In an advantageous development of the method according to the inventionthe monoaxially stretched plastics sheet bands used consist of olefinsof high molecular weight, which preferably contain polypropylenehomopolymers or polypropylene-polyethylene co-polymers.

According to another development of the method according to theinvention, the monoaxially stretched plastics sheet bands used arecomposed of polypropylene-polyethylene-diene terpolymers.

In a further development of the method according to the invention thematerial of the plastics sheet bands used contains a nucleation agent,which may preferably be based on an organometallic complex compound.

Such nucleation agents increase the number of crystallites in thematerial of the bands, which, as the Applicants have found, reduces thetendency to creep of the monoaxially stretched sheet bands used.

According to a last advantageous development of the method of theinvention, in preparation for the welding of the wound assembly thelatter is preheated, in the course of its production, in one or morepreheating devices, each interposed between two successive windingoperations, while in at least a part of the preheating devices thepartial wound assembly formed up to that point is advantageouslysuperficially smoothed.

The invention further relates to the strings which are produced by themethod according to the invention, and which have similar advantageousproperties to those of gut strings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 2 shows schematically a preferred plant for producing the racketstring. The plant comprises six serially disposed winders 1 to 6, whichare substantially identical in construction and of which only thewinders 1, 2 and 6 are shown in FIG. 2, a heated tubular weldingapparatus 7, and five preheating nozzles 8 to 12, each disposed betweentwo successive winders. The finished string 13, which is still hot,passes through a sizing apparatus 14 consisting of two grooved rollers,is drawn off by a draw-off apparatus 15 in the direction of the arrow16, and finally is wound up at 17.

FIG. 3 shows the winder 2 followed by the preheating nozzle 9 on alarger scale and in greater detail. Each of the winders comprises arotary star 18 having up to four pivotably mounted spools 21 providedwith draw-off brakes 19 and with adjustable guide eyes 20, for theplastics sheet bands 22 used. Each rotary star 18 is equipped with aseparate drive whose speed of rotation is continuously adjustable.

FIG. 4 shows schematically the winding process for each individual sheetband of the winding layers which constitute the two outermost layers 23and 24 of the string. This Figure will be further explained with the aidof the following description of the application of the invention.

BEST WAY OF APPLYING THE INVENTION

The method according to the invention for producing a racket string isfirst described below in greater detail with reference to one example.The manufacturing data, properties and spring stringing characteristicsof this string and also those of five other racket strings produced bythe method according to the invention can be seen in the Table at theend of the description or from FIG. 1.

In the example of embodiment which will be described (C₁ in the Table),bands 22 of a thickness of 35 μm and a width of 3 mm are used, whichconsist of a polypropylene-polyethylene-diene terpolymer extruded andthen cold stretched with a stretching ratio of 1:6.5 in the lengthwisedirection of the band. These bands 22 are wound onto spools 21, and thefirst four rotary stars 18, referring to the direction of advance 16,are each equipped with three spools 21 and the remaining two rotarystars 18 with two spools 21 each. This gives a winding sequence referredto below as 3/3/3/3/2/2. On commencement of production the bands 22 ofeach spool 21 are first guided through all the preheating nozzles 8 to12 lying in the draw-off direction 16 and also through the weldingapparatus 7, and are first drawn off together as a bundle by thedraw-off apparatus 15 at a constant speed in the range between 1 and 2meters per minute in the draw-off direction 16.

The drives of the rotary stars 18 and the heaters of the preheatingnozzles 8 to 12 and also that of the welding apparatus 7 are thenswitched on. For the present production example the rotary stars of thewinders 1, 3 and 5 are driven--viewing in each case in the oppositedirection to the draw-off direction 16--in the clockwise direction andthose of the winders 2, 4 and 6 are driven in the counterclockwisedirection.

The racket string is thus formed in the following manner: in the firstwinder 1, to which no initial bundle is fed, one of the wound sheetbands 22 is given a twisted structure, over which the other two bandsare applied are more or less uniform winding layers. The partial woundassembly 25 thus produced then passes through the first preheatingnozzle 8, in which it is heated to a temperature at which welding of thesheet bands does not yet occur but at which the elasticity of the bandmaterial is increased, whereby the superficial smoothing of the partialwound assembly 25 effected in the preheating nozzle 8 is facilitated.The initial bundle 26 passing out of the first preheating nozzle nowserves as a core for the winding layers consisting of sheet bands 22,which in the following winders 2 to 6 are wound helically andoverlappingly along concentric cylindrical surfaces.

FIG. 4 shows the process of winding respective sheet bands 27 and 28 ofwinding layers which constitute the two outermost layers 23 and 24 ofthe string.

The pitch of the windings is determined by the draw-off speed, measuredupstream of the welding apparatus 7, the speed of rotation of the rotarystar 18, and the diameter of the respective initial bundle. It increasesfrom one winding layer to the other from about 45° to 65°. The angle α(FIG. 3) at which the sheet band 22 advances to the winding point 29adjusts itself in each case to a constant value; the position of thewinding point 29 is determined by corresponding adjustment of theposition of the guide eye 20. Since as a result of the action of thedraw-off brakes 19 the sheet bands 22 are under tensile stress duringthe winding and because of the helical structure of the winding, a notinconsiderable contact pressure exists between the wound sheet bands 22and the initial bundle 25 serving as winding core, which in therespective upstream preheating nozzle was brought to a temperature belowthe welding temperature and superficially smoothed. These initialbundles already form in each case compact winding structures containingno noteworthy air inclusions.

The wound assembly 30 running off from the last winder 6 then passesthrough the welding apparatus 7, in which a temperature in the rangebetween 220° C. and 260° C. prevails. The already preheated woundassembly 30 is thereby brought to a temperature at which the sheet bands22, which within this assembly lie flat against one another and, asexplained above, are pressed against one another, are well weldedtogether, but at which their anisotropy produced by the cold working ofthe sheet material is not substantially destroyed, which means that thebreaking strength of the sheet bands used is not substantially reducedby this temperature treatment. In the present production example thisbrings about the result that the breaking strength of the finishedstring of 390N/mm², is lower than the breaking strength of the sheetbands used, which here amounts to 420N/mm².

The still warm string 13 running off from the welding apparatus 7 thenpasses through the sizing apparatus 14, in which in the presentproduction example it is brought to a diameter of 1.20 mm, whereupon itis drawn off by the draw-off apparatus 15 in an already substantiallycooled state and wound up at 17.

The finished string 13 has a spring constant characteristic as shown incurve C in FIG. 1. As can be seen, in the range of string prestressF_(v) of 200 to 300N, which is here mainly employed in practice, thevalues of the spring constants E.A. are substantially lower and withincreasing prestress F_(v) rise substantially less sharply than in thecase of comparable known plastics strings (curve B). The rise of thecharacteristic (curve C) is only slightly steeper than the rise of thecharacteristic of gut strings (curve A).

The data for five other production examples C₂, C₃, D₁, D₂, and E aretabulated in the Table at the end of the description.

In the production examples C₂, C₃, D₁, and D₂ the starting material isthe same polypropylene-polyethylene-diene terpolymer sheet material asin the production example C₁ described in detail above, with a differentstretching ratio of 1:6.5 (in the case of C₁, C₂ and C₃) and 1:8 (in thecase of D₁ and D₂) applied to the sheet band used. In addition, as avariation from the other examples, in example D₁ only five windinglayers are used instead of six. The polypropylene homopolymer used assheet material in Example E contains about 1% of the nucleation agent ofthe type PP-78040 based on an organometallic complex compound, assupplied by Gabriel-Chemie, Vienna.

The strings according to production examples C₂ and C₃, like the stringaccording to Example C₁, have a spring tension characteristic accordingto curve C in FIG. 1, the strings according to Examples D₁ and D₂ have acharacteristic according to curve D, and the string according to ExampleE has the curve correspondingly designated E.

On the strength of the results obtained in the production examples itcan be assumed that the spring tension characteristics of the racketstrings produced by the method according to the invention are dependentsubstantially only on the sheet band material used and on its stretchingratio and only slightly or not at all dependent on the dimensions, thenumber and the winding sequence of the bands used.

INDUSTRIAL EXPLOITABILITY

The racket string produced by the method according to the invention canbe employed with particular advantage for stringing tennis rackets. Theproperties of stringing of this kind are close to those of gutstringing. The manufacturing costs of the string according to theinvention are however substantially lower than those of a correspondinggut string.

                                      TABLE                                       __________________________________________________________________________    Racket Strings       Sheet Bands Used                                                         Breaking                   Breaking                                    Band   strength                   strength                           Exam-                                                                             Diameter                                                                           winding                                                                              σ.sub.R in                                                                         Thickness                                                                           Width                                                                             Stretching                                                                          σ.sub.R in                   ple in mm                                                                              sequence                                                                             N/mm.sup.2                                                                         Material                                                                            in μm                                                                            in mm                                                                             ratio N/mm.sup.2                         __________________________________________________________________________    C.sub.1                                                                           1.20 3/3/3/3/2/2                                                                          390                                                           C.sub.2                                                                           1.30 3/3/3/3/3/3                                                                          380  PP-PE-                                                                              35    3.0   1:6.5                                                                             420                                                     diene                                                                         ter-                                                                          polymer                                                  C.sub.3                                                                           1.40 3/4/4/4/4/3                                                                          370                                                           D.sub.1                                                                           1.25 3/4/4/3/3                                                                            395                                                           D.sub.2                                                                           1.30 3/4/4/4/2/2                                                                          402        30    2.8 1:8   464                                E.sup.                                                                            1.25 3/3/3/3/2/2                                                                          420  PP homo-                                                                            35    2.5 1:8   457                                                     polymer                                                                       with                                                                          nucle-                                                                        ation                                                                         agent                                                    __________________________________________________________________________

What is claimed is:
 1. Method of producing a string (13) for ballrackets, particularly for tennis rackets, wherein a plurality of windinglayers of helically wound plastics sheet bands (22) are applied to acontinuously fed core, one over the other, along at least approximatelyconcentric cylindrical surfaces, and the winding layers are joined toone another, characterized in that the plastics sheet bands (22) usedare made of plastic materials monoaxially stretched in the lengthwisedirection of the band, and that after the winding layers have been woundover the core the resulting wound assembly (30) is passed, while undertensile stress, through a welding zone (7) in which the plastics sheetbands (22) are welded together at a raised temperature.
 2. Methodaccording to claim 1, characterised in that the temperature in thewelding zone (7) is set at such a high level that, although a weldingunion is effected between the plastics sheet bands (22), the breakingstrength of the monoaxially stretched plastics sheet bands (22) used inthe winding layers are nevertheless not substantially reduced by thistemperature treatment.
 3. Method according to claim 2, characterised inthat, in cases where the string (13) is at least to a substantial extentconstructed of plastic sheet bands (22) of a uniform material, thetemperature in the welding zone (7) is so adjusted that the breakingstrength of the plastics sheet bands (22) is however not yetsubstantially reduced.
 4. Method according to claim 2, characterised inthat, in cases where the string (13) is at least to a substantial extentconstructed of plastics sheet bands (22) of a uniform material, thetemperature in the welding zone (7) is so adjusted that the reduction ofthe breaking strength of the plastics sheet bands (22) during thewelding operation leads to a breaking strength of the finished string(13) which is not more than 20%, and advantageously not more than 15%,below the breaking strength of the plastics sheet bands (22) used. 5.Method according to one of claim 1 or 2, characterised in that themonoaxially stretched plastics sheet bands (22) used are based onolefins of high molecular weight.
 6. Method according to claim 5,characterised in that the olefins of the band material containpolypropylene homopolymers.
 7. Method according to claim 5,characterised in that the olefins of the band material containpolypropylene-polyethylene copolymers.
 8. Method according to one ofclaim 1 or 2 characterised in that the monoaxially stretched plasticssheet bands (22) used are of polypropylene-polyethylene-dieneterpolymers of high molecular weight.
 9. Method of claim 1 or 2,characterised in that the material of the plastics sheet bands (22) usedcontains a nucleation agent.
 10. Method according to claim 9characterised in that the nucleation agent is based on an organometalliccomplex compound.
 11. Method of claim 1 or 2, characterised in that inpreparation for the welding of the wound assembly (30) the latter ispreheated, during its manufacture, in one or more preheating devices(8-12), each of which is interposed between two successive windingoperations.
 12. Method according to claim 11, characterised in that, atleast in a part of the preheating devices (8-12), the partial woundassembly (25) produced up to that point is superficially smoothed. 13.String (13) for ball rackets characterized in that it is produced by themethod of claim 1.